1. Field
The present invention relates to ferromagnetic amorphous alloy ribbon and to a magnetomechanical sensor element, also known as a marker or a tag, for use in an electronic article surveillance system, and an electronic article identification system, the sensor element comprising one or a plurality of rectangular strips based on an amorphous magnetostrictive material that vibrates in an alternating magnetic field mechanically at a resonant frequency varying with an applied static magnetic field, whereby the magnetomechanical effect of the marker is effectively utilized. The present invention is also directed to an electronic article surveillance system and an electronic article identification system utilizing such a sensor.
2. Background
Magnetostriction of a magnetic material is a phenomenon in which a dimensional change takes place upon application of an external magnetic field on the magnetic material. When the dimensional change is such that the material elongates upon its being magnetized, the material is termed “positive-magnetostrictive”. When a material is “negative-magnetostrictive”, the material shrinks upon its magnetization. Thus in either case, a magnetic material vibrates when it is in an alternating magnetic field. When a static magnetic field is applied along with the alternating magnetic field, the frequency of the mechanical vibration of the magnetic material varies with the applied static field through magneto-elastic coupling. This is commonly known as ΔE effect, which is described, for example, in “Physics of Magnetism” by S. Chikazumi (John Wiley & Sons, New York, 1964, page 435). Here, E(H) stands for Young's modulus, which is a function of the applied magnetic field H. The material's vibrational or resonance frequency fr is related to E(H) through the equation:fr=(½l)[E(H)/ρ]1/2  (1),where l is the length of the material, and ρ is the mass density of the material.
The magneto-elastic or magneto-mechanical effect described above is utilized in electronic article surveillance systems which were first taught in U.S. Pat. Nos. 4,510,489 and 4,510,490 (hereinafter, the '489 and '490 patents). Such surveillance systems are advantageous in that they offer a combination of high detection sensitivity, high operating reliability and low operating costs.
A marker in such systems is a strip, or a plurality of strips, of known length of a ferromagnetic material, packaged with a magnetically harder ferromagnet (material with a higher coercivity) that provides a static field termed as bias field to establish magneto-mechanical coupling. The ferromagnetic marker material is preferably a magnetostrictive amorphous alloy ribbon, since the efficiency of magneto-mechanical coupling in the magnetostrictive amorphous alloys is very high. The mechanical resonance frequency, fr, is determined essentially by the length of the alloy ribbon and the bias field strength, as Equation (1) above indicates.
When an interrogating signal tuned to the resonance frequency is encountered in the electronic article surveillance system, the marker material responds with a large signal field which is detected by a receiver in the system.
Several amorphous ferromagnetic materials were considered for electronic article surveillance systems based on magnetomechanical resonance described above in the '489 and '490 patents, including amorphous Fe—Ni—Mo—B, Fe—Co—B—Si, Fe—B—Si—C and Fe—B—Si alloys. Of the alloys, a commercially available amorphous Fe—Ni—Mo—B based METGLAS®2826 MB alloy was used extensively until accidental triggering, by a magnetomechanical resonance marker, of other systems based on magnetic harmonic generation/detection. This occurs because a magnetomechanical resonance marker used at that time sometimes exhibited non-linear BH characteristics, resulting in the generation of higher harmonics of the exciting field frequency. To avoid this problem, sometimes called a system “pollution problem,” a series of new marker materials were invented, examples of which were disclosed in U.S. Pat. Nos. 5,495,231, 5,539,380, 5,628,840, 5,650,023, 6,093,261 and 6,187,112. Although the new marker materials perform, generally, better than the materials utilized in the surveillance systems of the original '489 and '490 patents, somewhat better magnetomechanical performance was found in the marker materials disclosed, for example, in U.S. Pat. No. 6,299,702 (hereinafter, the '702 patent). The new marker materials require complicated heat-treatment processes to achieve desired magnetomechanical properties as disclosed, for example, in the '702 patent. Clearly, a new magnetomechanical marker material was needed which did not require such complicated post-ribbon fabrication processes, and the inventions of U.S. Pat. No. 7,205,893 (hereinafter, the '893 patent), 7,320,433 (hereinafter, the '433 patent) and 7,561,043 (hereinafter, the '043 patent) provided such a marker material with high magnetomechanical performance without causing the “pollution problem” that is mentioned above. A marker strip in accordance with the '702 patent is widely used for a marker with two strips, as is disclosed in U.S. Pat. No. 6,359,563. Due to the fact that the two strips have the same radius of curvature along the strip width direction since each of them was processed in exactly the same way, in accordance with the '702 patent, the two strips touch each other at many points on the strip surfaces, damping the magnetomechanical vibration on the strips, and hence reducing the effectiveness of the marker. This drawback was ameliorated with the '893, '433 and '043 patents. In maximizing the magnetomechanical resonance effect on which the '893, '433 and '043 patents are based, a new aspect controlling the effect has been discovered, which is the basis of the present invention. This invention, therefore, further enhances the magnetomechanical resonance effect utilized in the '893, '433 and '043 patents. Furthermore, there is a need for an effective electronic article surveillance system which utilizes such a marker.